Thermosettable resins are well known for their use in structural adhesives, and advanced composite materials used in electronic, architectural and aerospace applications. Articles prepared from these thermosettable resins, however, have been flammable, a characteristic that has seriously limited their use.
Many materials have been suggested for addition to synthetic resins to make them flame retardant. Various considerations must be taken into account in selecting a suitable flame retardant. Since the thermosettable resin is often required to perform under adverse conditions (for example, high temperature, high humidity or high stress conditions), the flame retardant must not seriously detract from the physical properties of the resin. It is also desirable that the flame retardant be inert and not degrade under process or use conditions. Premature degradation of the flame retardant within the resin network at high temperatures, high humidities or high stresses can initiate catastrophic matrix failure. In addition, the flame retardant should not interfere with process parameters. For example, a flame retardant that caused premature cure of the resin would be highly undesirable.
Chemical flame retardants that are utilized in many traditional applications such as textile treatments, surface coatings or low performance adhesives, have numerous shortcomings when used in thermosettable resins. For example, mineral fillers such as magnesium hydroxide (Mg(OH).sub.2), aluminum trihydrate (ATH), and poly(ammonium phosphate) are often used successfully as flame retardants for paints or reinforced adhesives. Such particulate flame retardants, however, are inadequate when used in thermosettable resins intended for use at elevated temperatures. These mineral fillers often require loading levels of 25% to 40% by weight to impart sufficient flame retardant performance to the thermosettable resin. At these high loadings, the dispersed mineral filler negatively impacts critical properties such as moisture uptake, ductility and resin strength. Moreover, the incompatibility of the mineral fillers in the thermoset resins compromises processing properties such as viscosity. In addition, mineral fillers are generally not suitable for making composite materials by resin transfer molding (RTM), where a thermosettable resin is injected into a fiber network such as carbon or glass fiber. When making composite articles by RTM, the composite fibers can filter out the dispersed flame retardant or prevent a uniform distribution of the flame retardant, thereby reducing its efficiency.
Unlike the aforementioned mineral fillers, red phosphorus is a particulate flame retardant that is efficient at lower loadings. However, red phosphorus has limited use in epoxy resin networks. Attempts to disperse red phosphorus into other resins result in cured resin articles that have uneven or aesthetically unappealing particulate dispersions. Moreover, like the mineral fillers, red phosphorus may decrease the strength of the cured resin. In addition, because of its particle size, red phosphorus may be filtered out by composite fibers when used to make composite articles by RTM.
To overcome the processing and high loading problems associated with mineral fillers, several classes of melt processable chemical flame retardants have been developed. In general, these chemical flame retardants consist of alkyl or aryl phosphate esters and organohalo compounds such as brominated epoxies. Resins comprising these materials are more easily processed than those with dispersed fillers. However, flame retardant loadings of 15% to 30% by weight are typically required to achieve adequate flame retardancy, and these flame retardants tend to lack the thermal and hydrolytic stability required for high performance composite systems. Although certain phosphate esters have improved thermal and hydrolytic stability, these materials still tend to plasticize the resin network to such a degree that upper temperature limits for article use are lowered. Organohalo compounds, such as decabromobiphenyl, typically exhibit higher moisture stability, but lack thermal stability. Moreover, when burned, the organohalo compounds release toxic and corrosive hydrohalogen gases. As a consequence, the use of halogen compounds is coming under increasing scrutiny because of environmental and health concerns. The teachings of Yoshioka in GB patent 1,487,632 further corroborates these points and addresses these issues by use of functional arylphosphate-phenylamides. Further discussion on conventional flame retardants can be found in the Handbook of Organophosphorus Chemistry, Engel, R., Chapter 14 by Marcel Dekker, (1992); the article of J. Green in the Journal of Fire Science, Volume 10, page 471 (1992); and the article by Fritz et. al. in the Angewandt Makromolekular Chemie, Volume 198, page 51 (1992).
Phosphonitrilic or phosphazene compounds have also been disclosed as an additive to synthetic resins, including thermosetting materials, to make them flame retardant. For the most part, these phosphazene compounds have been substituted by a functional group which enables the compound to react with the synthetic resin and become covalently bonded to the resin network.
In general, however, the incorporation of a functional phosphazene into a resin causes the resin network to be susceptible to thermal or hydrolytic degradation. Another major drawback is that the functional materials tend to negatively impact processing parameters, for example, resin viscosity and resin pot life.
The use of non-functional group-substituted cyclophosphazenes as flame retardants is also known for specific applications. For example, U.S. Pat. No. 3,865,783 describes the use of hexaphenoxycyclotriphosphazene as a flame retardant for polyesters that are to be used for melt spinning of fibers. U.S. Pat. No. 4,405,738 describes the use of cyclotri (or tetra) phosphazenes as flame retardant additives for polyesters to be used for melt spinning of fibers. U.S. Pat. No. 4,496,685 discloses an adhesive composition comprising an alpha-cyanoacrylate monomer and a phosphazene compound, including phenoxyphosphazene as a UV stabilizer. Japan Kokai No. 61/120850A discloses an epoxy resin composition used to seal semiconductors and other electronic circuit parts. The composition includes specific phosphazene compounds to reduce stresses due to thermal expansion occurring during cure of the epoxy resin, without sacrificing other properties, such as moisture resistance. A general discussion of the use of phosphazenes in heat resistant crosslinked matrix polymers, flame and heat resistant hydraulic fluids and lubricants, and chemosterilant insecticides can be found in the Apr. 22, 1968 issue of Chemical and Engineering News at pages 66-81.
It is also known that two or more types of flame retardants may be combined in a single system to give antagonistic, synergistic or additive effects (Encyclopedia of Polymer Science and Engineering, Volume 7, p. 182 (1987)). Antagonistic effects result when the different classes of flame retardants are less effective than expected from an additive model. Synergy exists when the combination of two or more flame retardant additives improves flame retardancy beyond what would be expected from a simple additive model. A well-known example of true synergy is found in the combination of antimony oxides with halogenated materials. True synergism is relatively rare (for a further discussion of synergy, see Flame Retardancy of Polymeric Materials, W. C. Kuryla, A. J. Papa, Volume 4, page 109 (1978)). Most commonly, two different types of flame retardants may show improved performance in an additive fashion.
For example, phosphorus-containing flame retardants have shown synergistic performance when combined with nitrogen-containing materials in cellulosic polymers. In other types of polymers, the efficiency of phosphorus-based flame retardants has been influenced by the overall ratio of phosphorus atoms to nitrogen atoms. Examples of materials which combine phosphorus and nitrogen sources for enhanced flame retardant performance are common in the art; in particular, these types of materials are often combined to produce intumescent compounds, which outgas during combustion to produce a physical flame retardant barrier of adherent char. Such materials are described in U.S. Pat. No. 5,182,388 wherein thermoplastic polymers are made flame retardant by the addition of derivatives of 2,4,6-triamino-1,3,5-triazine salified by the partial salt of an oxyacid of phosphorus. A polymer consisting of alternating derivatized phosphine oxide and 1,3,5-triazine repeating units is described in U.S. Pat. No. 5,158,999, also for imparting flame retardant properties to a thermoplastic but without intumescence.
It becomes apparent from the above discussion that there is critical need to develop a flame retardant additive for advanced composite materials that will provide flame retardancy without loss of performance properties. To date, no one flame retardant additive has been identified that can provide adequate flame retardant performance and ease of processing with minimal impact on composite properties.